Method for manufacturing a titanium part through initial beta forging

ABSTRACT

The invention relates to a method of fabricating a titanium alloy part, the method comprising:
         heating the part to a temperature T 1  so that the temperature of the part is substantially uniform, performing an initial forging operation on the part, followed immediately by quenching the part down to ambient temperature; and   heating the part to a temperature T 2 , followed by a final forging operation on the part at the temperature T 2  followed immediately by quenching the part, the final forging operation being suitable for giving the part its final shape;   the temperature T 1  being higher than the β-transus temperature of the alloy, the temperature T 2  being lower than the β-transus temperature, the only heating of the part to above the β-transus temperature being the heating to the temperature T 1 , the initial forging preceding the final forging, and the initial forging being performed as soon as the temperature of the part is substantially uniform, the method being characterized in that the quenching immediately following the initial forging is performed at a speed faster than 150° C./min, with the deformation ratio during the initial forging being greater than 0.7.

The present invention relates to a method of fabricating a part out of titanium alloy. More particularly, it relates to a method comprising:

-   -   heating said part to a temperature T₁ so that the temperature of         said part is substantially uniform, performing an initial         forging operation on said part with a deformation ratio that is         greater than 0.7, followed immediately by quenching said part         down to ambient temperature; and     -   heating said part to a temperature T₂, followed by a final         forging operation on said part at said temperature T₂ followed         immediately by quenching said part, said final forging operation         being suitable for giving said part its final shape.

Titanium alloys are used in high-tech applications, in particular for aviation turbines, in order to fabricate certain parts that are subjected to high levels of stress at high temperatures. Pure titanium exists in two crystallographic forms: the α phase, which is hexagonal and exists at ambient temperature, and the β phase, which is body-centered cubic and exists at temperatures above the so-called β-transus temperature, which is equal to 883° C. for pure titanium. On phase diagrams for titanium alloyed with other elements, the β phase is to be found above the β-transus temperature, and below that temperature there is equilibrium between the β phase and the α phase over an area that depends on the elements of the alloy. The αβ phase is constituted by a mixture of α phase and β phase. In particular, the alloying elements have the effect of causing the β-transus temperature to vary around 883° C. Developing a titanium alloy that possesses desired properties consists, in particular, in selecting alloying elements and in selecting the thermomechanical treatment to which the alloy is to be subjected.

For αβ or quasi-a alloys, such as the TA6V and Ti6242 alloys, the alloy is thus in the β phase above the β-transus temperature, and respectively in a state of equilibrium between the α and β phases, or essentially in the α phase at ambient temperature.

In the description below, the term “β domain” is used to designate the range of temperatures above the β-transus temperature, and the term “αβ domain” is used to designate the range of temperatures immediately below the β-transus temperature in which the α and β phases are in equilibrium.

By way of example, one present method of fabricating forged parts made of titanium alloys comprises a plurality of forging passes, all of which are performed in the αβ domain (the temperatures T₁ and T₂ are then both lower than the β-transus temperature). Such a forging range does not enable the macrostructure to be fully recyrstallized and refined. At the end of the forging, there remain large colonies of α phase nodules that are inherited from the alloy billet (semi-finished form). The term “colony of α nodules” is used to designate a group of one or more nodules presenting a preferred crystallographic orientation. These colonies contribute to reducing the ability of the part to withstand fatigue.

Another method of fabricating forged parts out of titanium alloys comprises a plurality of forging passes, these passes being performed in the αβ domain, with the exception of the large pass, which is performed in the β domain (the temperature T₁ is then lower than the β-transus temperature, while the temperature T₂ is higher than the β-transus temperature). This last pass at a higher temperature makes the part easier to shape. Nevertheless, this last forging pass takes place at a temperature higher than the β-transus temperature, so the entire microscopic structure of the part as obtained voluntarily during the earlier passes is erased. Furthermore, the alloy grains (microscopic structure) tend to become larger and the deformation ratio of the last forging pass is often not sufficiently great to encourage recrystallization, and thus refining, of the grains (since the part immediately prior to this last forging pass is already close to its final shape). Since the grains are larger, the mechanical properties of the part are diminished.

Furthermore, during the last forging pass, the dies that are used are complex in shape (in order to give the part its final shape), which gives rise to the part having a macrostructure that is not uniform (presence both of zones that are deformed little and of zones that are deformed considerably). This non-uniformity gives rise to large variations in mechanical behavior within the part.

The present invention seeks to remedy those drawbacks.

The invention seeks to propose a method of enabling a titanium alloy part to be obtained that possesses a structure that is more uniform and that possesses better mechanical properties, in particular in terms of ability to withstand fatigue.

This object is achieved by the facts that the temperature T₁ is higher than the β-transus temperature of the alloy, that the temperature T₂ is lower than the β-transus temperature, that the only time said part is heated above the β-transus temperature is when it is heated to the temperature T₁, that the initial forging precedes said final forging, the initial forging being performed as soon as the temperature of said part is substantially uniform, and that the quenching is performed at a speed faster than 150° C./min.

By means of these arrangements, the high deformation ratio of the part due to forging at a temperature that is sufficiently high serves to refine the microstructure (to obtain β grains of smaller size) and to erase the heredity of the part. Below the β-transus temperature, the part is constituted by β phase grains that are substantially equi-axial, since the part has not yet been deformed, given this is the first forging operation (the thickness of the part at this stage is substantially constant). Forging deforms those grains, which recrystallize into fine β grains. These small β grains themselves recrystallize into a fine needled a phase during quenching after forging. The part therefore does not have undesirable nodules of α phase at ambient temperature. The facts of subsequently quenching the part sufficiently fast and of subsequently not going back into the β domain enable this refined microstructure to be conserved, and avoids the grains growing.

Consequently, the microstructure of the alloy is refined and more uniform. The ability of the part to withstand fatigue is thus improved.

Furthermore, while detecting metallurgical defects by ultrasound, background noise is reduced. Such background noise is generated by non-uniformities in the microstructure. Since the structure is generally more uniform, it follows that background noise is diminished, and thus that any metallurgical defects in the part can be detected more finely and more easily.

The invention also provides an aviation part in the form of a body of revolution fabricated by a method of the invention.

The invention can be well understood and its advantages appear better on reading the following detailed description of an implementation given by way of non-limiting example. The description refers to the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating the method of the invention for fabricating a metal part out of titanium alloy;

FIG. 2A is a microphotograph of a titanium alloy heated to below the β-transus temperature;

FIG. 2B is an enlargement of the FIG. 2A microphotograph;

FIG. 3A is a microphotograph of a titanium alloy heated to above the β-transus temperature;

FIG. 3B is an enlargement of the FIG. 3 microphotograph;

FIG. 4A is a microphotograph of a titanium alloy heated to above the β-transus temperature and then deformed with a deformation ratio of 1; and

FIG. 4B is a microphotograph of a titanium alloy heated to above the β-transus temperature and then deformed with a deformation ratio of 2.5.

The method of the invention applies in general to a billet obtained by one or more melts of a titanium alloy, casting said alloy as an ingot, and then forging using a given thermodynamic cycle.

FIG. 1 is a diagram showing the steps of the method of the invention for fabricating a part out of titanium alloy. In the diagram, the abscissa axis represents increasing time t (no scale), and the ordinate axis represents temperature T in degrees Celsius, increasing from ambient temperature T_(A). The temperature of the part as a function of time t is represented in this diagram by a curve. In step 1, the part is heated to a temperature T₁ that is higher than the β-transus temperature for the alloy. The part is maintained at this temperature T₁ for a length of time that is long enough for the temperature of the part to be substantially uniform and equal to T₁ (step 1-1). This maintaining of temperature is represented by the plateau in step 1. There is no need to maintain the part for an excessively long length of time at the temperature T₁, since the transformation of the α phase into the β phase takes place immediately on passing above the β-transus temperature. Furthermore, maintaining the part for too great a length of time above the β-transus temperature would lead to grains becoming larger, which is harmful, since it reduces the mechanical performance of the final part. Forging must therefore be performed as soon as the temperature of the part is substantially uniform and equal to T₁, and as soon as possible given the capabilities of the industrial process.

The microstructure difference between a titanium alloy heated to above the β-transus temperature and the same alloy heated to below the β-transus temperature is shown by comparing FIGS. 2A and 2B with FIGS. 3A and 3B.

FIG. 2A is a photograph taken by a microscope of a titanium alloy heated to a temperature that is immediately below the β-transus temperature, and without being subjected to forging (the β-transus temperature for this alloy is 1001° C.). FIG. 2B is an enlargement of the zone in FIG. 2A that is outlined by a rectangle. In FIG. 2B, it can be seen that oriented structures are present in the alloy, specifically oriented fibers constituted by substantially parallel needles 10 (elongate grains).

FIG. 3A is a photograph taken using a microscope showing the same titanium alloy as that shown in FIG. 2A, but after heating to a temperature immediately above the β-transus temperature, and without being subjected to forging. FIG. 3B is an enlargement of the zone of FIG. 3A that is outlined by a rectangle. It can be seen that after passing above the β-transus temperature, the oriented fibers disappear and the structure is more isotropic. As soon as the alloy temperature exceeds the β-transus temperature, a phase is transformed into β phase, thereby giving rise to equi-axial recrystallization of the microstructure, accompanied by an increase in grain size. The stresses that exist in the part prior to heating about the β-transus temperature are very largely eliminated. The microstructure and the state of the alloy is thus more appropriate for being subjected to the forging operation.

As explained above, it is necessary for the entire part to be at a temperature that is higher than the β-transus temperature during the forging operation, as happens once all of the zones of the part are substantially at the temperature T₁. The part is then forged at a temperature that is substantially equal to T₁ so as to give it an intermediate shape that approaches its final shape (step 1-2).

During this initial forging operation, the deformation ratio is greater than 0.7. The deformation ratio T_(d) is defined as being the logarithm of the ratio of the thickness H_(i) of the part prior to deformation and its thickness H_(f) after deformation:

$T_{d} = {{Log}\left( \frac{H_{i}}{H_{f}} \right)}$

If the part is not deformed, (i.e. H_(f)=H_(i)), then the deformation ratio T_(d) is equal to 0.

Advantageously, the deformation ratio is greater than 1. Preferably it is greater than 1.6. A higher deformation ratio gives rise to greater refining of the microstructure (reduction of grain size), thereby improving the resistance of the part to fatigue. These microstructure differences can be seen in FIGS. 4A and 4B, which are photographs taken using a microscope and which show a Ti6242 alloy after forging in the p domain with a deformation ratio of 1 and with a deformation ratio of 2.5, respectively. Tests performed by the inventors on these samples have shown that the lifetime of such a Ti6242 alloy goes from 78,000 cycles (at 772 MPA) for a deformation ratio equal to 1, to 130,000 cycles for a deformation ratio equal to 2.5.

Ideally, the initial forging operation above the β-transus temperature should be implemented using dies such that the shape of the part after forging is as close as possible to the final shape of the part, so as to minimize the stresses generated by the subsequent final forging operation. Furthermore, care can be taken to use dies that are of simple shape (e.g. a frustoconical die, in a flat stack, or of a diabolo shape) so as to enable material to flow freely throughout the mold and prevent any material becoming trapped in cavities during the forging operation.

For example, immediately after this initial forging, the shape of the part is of the frustoconical or diabolo type.

Once the part has been subjected to the forging operation in the β domain, it is subjected to quenching (step 1-3) from the forging temperature T₁ down to ambient temperature at a speed faster than 150° C./min (degrees Celsius per minute). This rapid quenching serves to conserve a fine microstructure for the part (fine grains) and thus to optimize the mechanical characteristics of the part, in particular its elastic limit, as has been verified during mechanical testing undertaken by the inventor.

Advantageously, the quenching is performed at a speed lying in the range 200° C./min to 400° C./min. Even more advantageously, the quenching is performed at a speed substantially equal to 250° C./min, where tests carried out by the inventors have shown that the mechanical characteristics are best optimized at this quenching speed. Quenching is preferably performed in water.

After quenching, the part is heated to a temperature T₂ that is lower than the β-transus temperature (corresponding to step 2 in FIG. 1). At the temperature T₂, the alloy is thus in the αβ domain, and the microstructure of the alloy is not modified. Any fibers (needle structures) produced during initial forging are thus conserved. Once the part has been heated to the temperature T₂ (step 2-1), the final forging operation is performed (step 2-2).

This final forging is followed by quenching (step 2-3) down to ambient temperature T_(A). This quenching serves to optimize the mechanical characteristics of the part, and in particular its elastic limit.

Under certain circumstances, the method of the invention may include one or more intermediate forging passes, all in the αβ domain (and thus at a temperature lower than the β-transus temperature), which passes are performed after the initial forging and before the final forging.

Under certain circumstances, it may be advantageous for the final forging to be followed by a tempering operation in the αβ domain. This forging tempering (step 3 in FIG. 1) in the αβ domain is thus performed at a temperature that is lower than the β-transus temperature. Thus, once the part has been quenched at the end of the final forging (step 2), the part is heated to a temperature T₃ (step 3-1), and is then cooled without quenching (step 3-2) down to ambient temperature. For the Ti6242 alloy, the temperature T₂ is approximately equal to 1000° C., and the temperature T₃ is equal to 595° C. There is no forging of the part during this tempering operation, so the part does not change shape. This tempering also serves to reduce the residual stresses generated in the part by the final forging operation.

Solution annealing of the part between final forging and tempering (at a temperature lying in the range T₂ and T₃) is pointless (since the final forging is in the domain and is therefore less severe), or might even be harmful.

Various titanium alloys may be subjected to the above-described method of the invention. For example, the titanium alloy used is an alloy of the αβ or quasi-α titanium family. In particular, the alloy may be TA6V or Ti6242 (TA6Zr4DE). By way of example, these alloys are used in aviation turbines.

Tests performed by the inventors on Ti6242 alloys show that a part obtained by a method of the invention possesses better fatigue properties than does a part obtained by a prior art method.

The part fabricated by a method as described above may be a disk for an aviation turbine, for example. By way of example, the part may be a drum for an aviation turbine.

Under certain circumstances, depending on the nature of the titanium alloy and on the type of part being treated, a portion only of the part is heated to above the β-transus temperature and is subjected to the method of the invention. Such forging is then referred to as upset forging. 

1. A method of fabricating a part out of titanium alloy, the method comprising: heating said part to a temperature T₁ so that the temperature of said part is substantially uniform, performing an initial forging operation on said part, followed immediately by quenching said part down to ambient temperature; and heating said part to a temperature T₂, followed by a final forging operation on said part at said temperature T₂ followed immediately by quenching said part, said final forging operation being suitable for giving said part its final shape; said temperature T₁ being higher than the β-transus temperature of said alloy, said temperature T₂ being lower than the β-transus temperature, the only heating of said part to above the β-transus temperature being the heating to the temperature T₁, said initial forging preceding said final forging, and said initial forging being performed as soon as the temperature of said part is substantially uniform, said method being characterized in that said quenching immediately following said initial forging is performed at a speed faster than 150° C./min, with the deformation ratio during said initial forging being greater than 0.7.
 2. A method according to claim 1, characterized in that said deformation ratio is greater than
 1. 3. A method according to claim 1, characterized in that said deformation ratio is greater than 1.6.
 4. A method according to any one of claims 1 to 3, characterized in that said quenching is performed at a speed substantially equal to 250° C./min.
 5. A method according to any one of claims 1 to 4, characterized in that said final forging is followed by an αβ phase tempering operation.
 6. A method according to any one of claims 1 to 5, characterized in that said titanium alloy is an alloy of the αβ or quasi-α titanium family.
 7. A method according to any one of claims 1 to 6, characterized in that said titanium alloy is selected from TA6V alloy and Ti6242 alloy.
 8. A method according to any one of claims 1 to 7, characterized in that said shape of the part immediately after the initial forging is of the frustoconical or diabolo type.
 9. A method according to any one of claims 1 to 8, characterized in that said part is a body of revolution for an aviation turbine.
 10. A method according to any one of claims 1 to 8, characterized in that said part is a disk for an aviation turbine.
 11. A method according to any one of claims 1 to 8, characterized in that said part is a drum for an aviation turbine. 